The invention relates to a device for optically scanning a surface, such a device comprises a radiation source-detection unit and a focusing unit for focusing radiation supplied by the radiation source-detection unit on a surface. The radiation source-detection unit includes a radiation source for supplying a radiation beam, a grating having a first grating structure for forming at least a deflected sub-beam from the radiation beam supplied by the radiation source or from a beam reflected by the surface, a radiation-sensitive detection system for converting radiation reflected by the surface into electric signals, an element for forming a monitor beam from the beam supplied by the radiation source, and a monitor detector for converting the monitor beam into an electric signal for controlling the radiation source. The invention also relates to a radiation source-detection unit for use in such a device.
A device of the type described in the opening paragraph is known from European Patent Application No. 0 372 629, which corresponds to U.S. Pat. No. 4,924,079. That device can be used for scanning a record carrier by means of a radiation beam. A three-spot grating is placed in the radiation beam of the device. In transmission, this grating forms two first-order diffracted sub-beams from the radiation beam. Together with the on-going non-deflected main beam, the two sub-beams are focused by an objective system to three spots on the record carrier. The main spot formed by the main beam is used for writing or reading information in the tracks of the record carrier. The side spots formed by the side beams are used for generating a tracking signal with which a circuit is controlled which keeps the main spot on the track to be scanned.
When information is being written in the record carrier, it is desirable that the power in the main beam is maintained accurately at a given value, referred to as the write power. To this end a reflector is arranged around the grating, which reflector reflects a part of the radiation from the radiation source falling outside the grating, referred to as the border radiation, to a monitor detector arranged beside the radiation source. The signal from the monitor detector is subsequently used for controlling the power supplied by the radiation source. A drawback is that the ratio between the border radiation power and the main beam power is dependent on the radiation source used, so that the monitor detector must be calibrated for each device.